1. Field of the Invention
This invention relates to silicas and silica colloids and products derived from the silica colloid. This invention also concerns a process for the production of silicas and silica colloids.
2. Description of Relevant Art
Precipitation of silica involves a complex array of phenomena involving a number of chemical reactions. Polycondensation and flocculation, which often compete with each other, are particularly important phenomena affecting the characteristics of precipitated silica.
Moreover, the term precipitated silica covers a product which is variable in its morphology. The ultimate particles or elementary pearls of a precipitated silica, for instance, may vary widely in size, shape, and other surface characteristics. Some of these morphological characteristics of silica may be detected with an electron microscope (see Ralph K. Iler-The Chemistry of Silica-John WILEY & Sons (1979) (hereinafter "ILER"), page 465).
Variations in size of ultimate particles of silica may result in different degrees of association among the ultimate particles. As the particles group together, they form aggregates (see ILER-supra at pages 476-477) and groups of weaker bonds. Aggregation results in a wide variety of morphologies of amorphous type silicas. The aggregates may be characterized by their size, their form factor and their surface area.
Thus, there is not one silica but an infinity of silicas, the behavior of which cannot be foreseen, especially as surface chemistry is an important characteristic of silicas which significantly affects their chemical and physical properties.
When producing silicas, it is necessary to consider a large number of parameters affecting the characteristics of the final product, such as the levels of concentration of various reactants, pH value, reaction time, and reaction temperature. Attempts have long been made to isolate the various chemical reactions involved in the formation of silicas and their effect on the final product. U.S. Pat. No. 2,731,326, for instance, discloses first forming a sol by the addition of a solution of sulfuric acid to a silicate solution, then heating the sol in order to increase the size of the particles to about 5 to 7 nm. After particles are formed, a solution of acid and a solution of silicate are simultaneously added, at a constant pH value, to provide for precipitation and to deposit the remainder of the silica on the precipitate (See ILER, supra at page 558).
Similarly, U.S. Pat. Nos. 3,954,944 and 4,127,641 provide for adding simultaneously sulfuric acid and a solution of a sol, maturing the sol, flocculating the sol with the introduction of the alkaline electrolyte, maturing the flocculation product and further adding acid alone or acid and silicate.
It should be understood that the methods of producing silicas are complicated and that the properties of the products themselves are still difficult to appreciate. These problems make it difficult to discern how various production parameters affect the characteristics of the final product. Thus, in spite of efforts which have been made over a long period of time, it has not been possible heretofore to provide a process which isolates the chemical reactions involved in the production of silicas or to control them, even at the level of the starting colloid.